The present invention relates to acceleration cavities for heavy ions and, more particularly, to a method and an apparatus for making the acceleration cavities superconductive at radiofrequency.
The application of superconductivity to acceleration cavities at radiofrequency allows beams of particles or heavy ions to be accelerated to otherwise unpredictable energy levels with a low power consumption. Niobium is a superconducting material which provides the greatest quality factor and allows the highest acceleration fields to be reached in an accelerating cavity due to its high critical temperature (T.sub.c), its high critical field, its low resistivity under normal conditions, and its ability to resist surface oxidation.
The construction of cavities using niobium as the superconducting material, however, involves a highly sophisticated technology since the superconductivity property of niobium is strongly affected by impurities of even diminutive concentration. This is why cavities of copper are preferably made superconductive by electrodeposition of lead (electroplating), which has, however, a reduced efficiency since radiofrequency loss is proportional to the exponential of the ratio -T.sub.c /T; and the T.sub.c of niobium is 9.25.degree. K., while the T.sub.c of lead is 7.2.degree. K. Moreover, with regard to acceleration fields, those provided by niobium are higher than those provided by lead; the critical magnetic field H.sub.c of lead is 800 gauss at 0.degree. K. whereas that of niobium is 2000 gauss at 0.degree. K.
Therefore, cavities for heavy ions, and in particular quarter-wave cavities or resonators (QWR) of niobium, have been provided only by electron-beam soldering of rolled sheets of niobium or composite sheets of copper and niobium produced by explosion. The results at radiofrequency confirm the absolute superiority of niobium over lead, but are heavily attendant with high cost. Moreover, the high number of electron-beam soldering required causes residual radiofrequency loss and makes the mechanical construction process more complex.